The present invention relates to a method of preparing Form II crystals of clarithromycin; and to novel intermediates used in said method.
Clarithromycin, 6-O-methylerythromycin A, is a semisynthetic macrolide antibiotic of formula (I) which exhibits strong antibacterial activity toward a wide range of bacteria inclusive of gram positive bacteria, some gram negative bacteria, anaerobic bacteria, Mycoplasma, Chlamidia and Helicobacter pylori, and in virtue of its high stability in the acidic environment of the stomach, it can be orally administered to treat many infectious diseases, and also to prevent recurrence of ulcer when used in a combination with other medicines: 
It has been reported that clarithromycin exists in at least three distinct crystalline forms, xe2x80x9cForm 0xe2x80x9d, xe2x80x9cForm Ixe2x80x9d and xe2x80x9cForm IIxe2x80x9d (International Publication Nos. WO 98/04573 and WO 98/31699). The crystal forms can be identified by infrared spectroscopy, differential scanning calorimetry and powder X-ray diffraction spectrophotometry. Form II, which is thermodynamically more stable than Form I, is used in the drug formulations currently on the market.
Various methods for preparing clarithromycin have been reported, e.g., in EP Patent Nos. 0,147,062, 0,158,467, 195,960 and 260,938; and U.S. Pat. Nos. 4,990,602, 5,837,829, 5,929,219, 5,892,008, 5,864,023 and 5,852,180. The most widely used methods use an erythromycin A 9-oxime derivative as an intermediate, which are described below.
Method 1) disclosed in EP Patent No. 0,158,467 comprises the steps of: protecting the oxime hydroxy group as well as the 2xe2x80x2-hydroxy group and 3xe2x80x2-dimethylamino group of erythromycin A 9-oxime with a benzyl group and benzyloxycarbonyl groups, respectively; methylating the 6-hydroxy group; and removing the protecting groups and the oxime group to obtain clarithromycin. However, this method requires the use of excessive amount of corrosive and toxic benzyloxycarbonyl chloride and is not amenable to commercialization due to the involvement of hydrogenolysis steps, which are difficult to use on a commercial scale.
Method 2) disclosed in EP Patent No. 0,195,960 comprises the steps of: protecting the oxime hydroxy, 2xe2x80x2-hydroxy and 3xe2x80x2-dimethylamino groups of erythromycin A 9-oxime with benzyl groups; methylating the 6-hydroxy group; and removing the protecting groups and the oxime group to obtain clarithromycin. However, this method suffers from several problems occurred in removing the protecting group.
Method 3) disclosed in EP Patent No. 0,260,938 comprises the steps of: protecting the oxime hydroxy group of erythromycin A 9-oxime with a benzyl or substituted benzyl group; protecting the 2xe2x80x2- and 4xe2x80x3-hydroxy groups with silyl groups; methylating the 6-hydroxy group; and removing the protecting groups and the oxime group to obtain clarithromycin. In this method, however, the oxime-protecting group also is removed by way of conducting a hydrogenolysis reaction, which is not suitable for mass-production.
Further, method 4) disclosed in U.S. Pat. No. 5,837,829 comprises the steps of: protecting the oxime hydroxy group and 2xe2x80x2- and 4xe2x80x3-hydroxy groups of erythromycin A 9-oxime with silyl groups; methylating the 6-hydroxy group; and removing the protecting groups and the oxime group to obtain clarithromycin. However, this method requires an extreme anhydrous condition in the methylation step due to the instability of the 9-oxime silyl group toward water, and also has the difficulty of handling hazardous of sodium hydride.
In addition, method 5) disclosed in U.S. Pat. No. 4,990,602 comprises the steps of: protecting the oxime hydroxy group of erythromycin A 9-oxime with a ketal derivative; protecting the 2xe2x80x2- and 4xe2x80x3-hydroxy groups with silyl groups; methylating the 6-hydroxy group; and removing the protecting groups and the oxime group to obtain clarithromycin. Although this method gives a relatively high yield of 45 to 50% and high selectivity of 90% in the methylation step, it requires the use of a large excess amount (2.3 to 10 equivalents) of a oxime protecting agent.
Thus, prior art methods such as methods 1) to 5) have many problems that must be solved to obtain an improved process for making clarithromycin. Moreover, the clarithromycin product obtained by the above methods is not a pharmaceutical grade clarithromycin, which must be a pure crystal Form II of clarithromycin, but non-pharmaceutical grade clarithromycin in aspect of purity and crystallinity. Accordingly, there is required a further purification step and a special crystallization step to convert non-pharmaceutical grade clarithromycin to pure Form II crystals of clarithromycin used in the current drug formulations.
There have been reported several methods of preparing Form II crystals from non-pharmaceutical grade clarithromycin. For example, Form 0 or Form I crystals of high purity are heated under a vacuum at a temperature ranging from 70 to 110xe2x96xa1 for a prolonged period of time to prepare Form II crystals (see International Publication Nos. WO 98/04573 and WO 98/31699), but this method has the problem of low productivity.
Alternatively, Form II crystals may be obtained by recrystallizing Form I crystals from chloroform/isopropyl ether (see Merck Index 12th ed., pp. 395), or by recrystallizing Form I crystals from an organic solvent or a mixture of an organic solvent and water in a moderate yield (see International Publication No. WO 98/04574). In these methods, since the conversion of Form I to Form II does not accompany purity enhancement, high purity Form I crystals must be prepared in advance from crude clarithromycin, at the expense of reduced clarithromycin yield and high manufacturing cost.
Accordingly, there has continued to exist a need to develop a high yield process for preparing high purity Form II crystals of clarithromycin.
Accordingly, it is a primary object of the present invention to provide a high yield process for preparing high purity Form II crystals of clarithromycin.
It is another object of the present invention to provide novel intermediates prepared in said method.
In accordance with one aspect of the present invention, there is provided a method of preparing Form II crystals of clarithromycin (formula I) comprising the steps of:
(a) treating non-pharmaceutical grade clarithromycin with methanesulfonic acid in a mixture of a water-miscible organic solvent and water to obtain crystalline clarithromycin mesylate trihydrate of formula (II); and
(b) neutralizing the crystalline clarithromycin mesylate trihydrate obtained in step (a) with aqueous ammonia in a mixture of a water-miscible organic solvent and water; wherein non-pharmaceutical grade clarithromycin refers to clarithromycin of any purity or of any stage of crystalline including a crude product obtained from a manufacturing process thereof: 
In accordance with another aspect of the present invention, there is provided an improved process for preparing non-pharmaceutical grade clarithromycin which comprises the steps of:
protecting the 9-oxime hydroxy group of erythromycin A 9-oxime of formula (IV) or a salt thereof with a tropyl group and 2xe2x80x2- and 4xe2x80x3-hydroxy groups with trimethylsilyl groups to obtain 2xe2x80x2,4xe2x80x3-O-bis(trimethylsilyl)erythromycin A 9-O-tropyloxime of formula (IIIb);
reacting 2xe2x80x2,4xe2x80x3-O-bis(trimethylsilyl)erythromycin A 9-O-tropyloxime with a methylating agent to obtain 2xe2x80x2,4xe2x80x3-O-bis(trimethylsilyl)-6-O-methylerythromycin A 9-O-tropyloxime of formula (IIIc); and
removing the protecting groups and the oxime group of 2xe2x80x2,4xe2x80x3-O-bis(trimethylsilyl)-6-O-methylerythromycin A 9-O-tropyloxime: 
In accordance with still another aspect of the present invention, there are provided crystalline clarithromycin mesylate trihydrate of formula (II); and erythromycin A 9-O-tropyloxime derivative of formula (III): 
wherein,
R1 is hydrogen or methyl group; and
R2 is hydrogen or trimethylsilyl group (if R1 is methyl group, R2 is trimethylsilyl group).